Reciprocating piston and cylinder arrangements in internal combustion engines, pumps, and the like, typically require a seal between the piston and the cylinder so that a pressure difference can exist between the two ends of the piston. This pressure difference permits the piston to provide a fluid pumping action useful in many things, including pumps and internal combustion engines. Sufficiently sealed piston and cylinder arrangements can be used in two, four, or multi-cycle internal combustion engines, free-piston engines, caloric engines, turbochargers, superchargers, compressors, pumps, and vacuums, for example.
It is to be appreciated that the reference herein to a “cylinder” is not limited to a chamber having a cylindrical shape or circular cross-section. Instead, the term cylinder refers to any chamber or cavity that receives a piston having an outer shape adapted to allow the piston to seal against the sidewall of the cylinder but at the same time permit the piston to slide back and forth reciprocally within the cylinder in a pumping motion.
Engine cylinders may include one or more intake ports and one or more exhaust ports that, collectively, permit gases to flow into, and out of, the engine cylinder, respectively. Engine valves, such as poppet valves, may be used to selectively open and close the intake and exhaust ports. The selectively timed opening and closing of the intake and exhaust valves, in conjunction with the pumping motion of the engine pistons and the introduction of fuel, may provide an air/fuel charge to the engine cylinder for combustion and removal of the spent charge exhaust gases from the cylinder after combustion.
Existing internal combustion engine pistons used for Otto cycle or Diesel cycle operation, for example, typically have a generally cylindrical shape. More specifically, the typical Otto or Diesel cycle engine piston may have a generally smooth cylindrically shaped skirt with a circular cross-section that includes circumferential recesses to receive one or more sealing piston rings. The piston and piston ring assembly may slide reciprocally within a cylinder between top dead center and bottom dead center positions. The interface of the piston rings with the cylinder wall may be lubricated with engine oil, for example.
Internal combustion engines almost universally require liquid lubricant, such as engine oil, to lubricate the interface between the piston and the cylinder within which it moves back and forth in a reciprocal motion. Lubrication systems are usually mission critical and the failure of a lubrication system can be catastrophic. The need for a piston lubricant brings with it many disadvantages. The lubricant wears out and becomes contaminated over time, and thus requires replacement, adding expense and inconvenience to engine operation. Many lubricants require pumps and passages to reapply the lubricant to moving parts, such as the engine pistons. Pumps and passages, and other elements of an active lubrication system need to operate correctly and require seals between interconnected elements. Lubrication system leaks naturally occur as seals deteriorate over time, and pumps leak and wear out, adding still further maintenance expense and inconvenience to engine operation. Leaks can also permit lubricant to enter the combustion chamber, interfering with combustion, and fouling injectors and spark or glow plugs. Lubricant in the combustion chamber can also result in unwanted exhaust emissions. Leaks can also result in the contamination of the lubricant with combustion by-products. All of the foregoing issues are attendant to the use of lubricated pistons, and all add failure modes and maintenance costs. Accordingly, there is a need for internal combustion engines that depend less, or not at all, on piston lubrication.
While embodiments of the invention are not limited to use in internal combustion engines, such engines may benefit from the invention since they routinely use piston and cylinder arrangements in which the piston is sealed with respect to the cylinder using one or more vertically spaced sealing piston rings disposed about the outer surface of the piston skirt. Many other devices besides internal combustion engines and pumps may include moving elements which require that a seal be formed between them. Embodiments of the invention may be used for these applications as well.